A. Field of the Invention
The invention relates generally to optical transmission systems employing optical modulators, more particularly, to optical modulation comprising phase, amplitude, and data modulation.
B. Background of the Invention
Optical modulation is presently used to encode data packets onto light for transmission in an optical communication system. In conventional systems, data is first modulated into a predetermined protocol by a data modulation device. Data modulation, for example, may comprise a device for converting electrical data packets into optical data packets. To reduce noise and other parameters, simple xe2x80x9c1xe2x80x9d xe2x80x9c0xe2x80x9d optical data packets must be further modulated. Amplitude modulation (AM) and phase modulation (PM) are commonly used to further modulate the optical data packet.
Mach-Zehnder type optical modulators are commonly used to provide AM modulation and PM modulation. AM modulators are often called xe2x80x9cintensity modulatorsxe2x80x9d as the rise and fall of the amplitude of the optical signal corresponds to the intensity of the optical signal emitted from the optical modulator. The term xe2x80x9cchirpxe2x80x9d generally refers to the amount of frequency modulation or PM modulation of the optical signal emitted from the optical modulator. Hence chirp intensity modulators, including some Mach-Zehnder type optical modulators, provide both AM and PM modulation of an optical signal. One such chirp intensity modulator is the JDS Uniphase(copyright) 10.66 Gb/s chirped return to zero (RZ) pulse generator.
Conventional optical communication systems first data modulate an optical signal, then use an AM/PM modulator device to further modulate the data modulated optical signal. One problem with conventional optical systems employing separate data modulators and AM/PM modulators is that multiple clock sources are used which have to be properly synchronized to be able to decode the data at a receiver. Clock synchronization adds to the complexity of conventional systems. Hence, a need exists for a device that provides data modulation, AM modulation, and PM modulation of an optical signal all within a single device.
The present invention is intended to improve on one or more of the problems described above, and other problems with the prior art.
According to a first aspect of the present invention, an optical modulator is provided comprising a first optical signal path, a second optical signal path, an offset waveguide electrode positioned between the first optical signal path and the second optical signal path, and an RF data modulator driver connected to the offset waveguide electrode. The RF data modulator driver comprises an AND GATE having a first input connected to a clock and a second input connected to a data source. The RF data modulator driver is adapted to provide a data modulated RF signal along the offset waveguide electrode causing an electric field to be generated along the first optical signal path and the second optical signal path. The electric field performs amplitude modulation, data modulation, and phase modulation of an optical signal propagating along the first optical signal path and the second optical signal path. Preferably, the clock is biased to adjust the rise and fall time of the output of the AND GATE.
According to another aspect of the present invention, a method of modulating an optical signal is provided comprising the steps of providing a first electric field in a first optical signal path by at least an offset waveguide electrode, providing a second electric field in a second optical signal path by at least the offset waveguide electrode, transmitting an optical signal along the first optical signal path and the second optical signal path, ANDing a clock source and a data source to provide a data modulated RF signal on the offset waveguide electrode, amplitude modulating the optical signal via the first electric field and the second electric field, and phase modulating the optical signal via the first electric field and the second electric field. The second optical signal path is positioned adjacent to the first optical signal path. The offset waveguide electrode is positioned between the first optical signal path and the second optical signal path. The magnitude of the electric field of the first electric field in the first optical signal path is greater than the magnitude of the electric field of the second electric field in the second optical signal path.
Preferably, the steps of providing a first electric field and providing a second electric field are performed by an X-Cut LiNbO3 type optical modulator. More preferably, an optical signal is formatted to a data transmission protocol, wherein the step of providing an optical signal along a first optical signal path and a second optical signal path provides a formatted optical signal on the first optical signal path and the second optical signal path.
According to another aspect of the present invention, an optical data modulator is provided comprising an electrode positioned adjacent to an optical path, and an electrical data modulator driver. The electrical data modulator driver comprises a first input connected to a clock source, a second input connected to a data source, and an output connected to the electrode. The electrical data modulator drier provides a data modulated electrical signal propagating along the electrode. The electrode data modulates an optical signal propagating along the optical path via at least one electric field responsive to the data modulated electrical signal. Preferably, the optical signal propagating along the optical path is modulated to a chirped return to zero (RZ) protocol.